Japanese Patent Application Laid-Open No. 2005-258747 discloses a fire alarm system using a sound wave. This system estimates a density of smoke existing in a monitoring space and temperature of the monitoring space, and determines existence or nonexistence of a fire, based on a fluctuation of a sound pressure at a time when a sound wave sent from a sound source is received through the monitoring space and a fluctuation of a time until the sound wave is received.
However, due to a change with the passage of time of a sensor for receiving the sound wave and a characteristic change depending on temperature and humidity, the received sound pressure undergoes a lot of changes. Thus, it becomes impossible to distinguish this change of the sound pressure and an attenuation of the sound pressure depending on a smoke existing in the monitoring space, and the system, which estimates the smoke density based on the sound pressure of the received sound wave, has a problem that it is impossible to detect the attenuation accurately.